Data storage systems that implement an abstraction layer between physical storage resources and logical address space often reserve some physical capacity to improve input/output (I/O) efficiency for random write-intensive workloads. Such systems also often utilize storage reduction techniques such as compression, de-duplication, and/or copy-by-reference to lower the ratio of physical capacity used per unit of logical data stored. Implementing both a guaranteed retention level and an opportunistic retention level for data storage in such systems may enable previously reserved physical capacity to be used to store data that is opportunistically retained, and/or may allow physical space conserved via storage reduction techniques to be used to store more data.